U.S. patent number 5,335,249 [Application Number 08/099,058] was granted by the patent office on 1994-08-02 for method and apparatus for spread spectrum communications.
This patent grant is currently assigned to Seattle Silicon Corporation. Invention is credited to Kenneth L. Astrof, Thomas E. Krueger.
United States Patent |
5,335,249 |
Krueger , et al. |
August 2, 1994 |
Method and apparatus for spread spectrum communications
Abstract
A method and apparatus for providing spread spectrum
communications. The apparatus starts with designated spread
spectrum communications parameters, such as default transmitted
power level, pseudo-noise (PN) sequence, PN sequence length, and
frequency channel, having values which are chosen to ensure that
spread spectrum communications can be carried out between a first
station and a second station. The apparatus of the invention then
changes the values of the spread spectrum communications parameters
to the minimum values necessary to provide a given level of
communications reliability. If the conditions facing the spread
spectrum communication system change so that communications
reliability falls below the given level, the apparatus resets the
values of the spread spectrum communications parameters to the
default levels and begins again to change their values to provide
the given level of communications reliability. If the apparatus
establishes that communications can reliably be carried out with
narrowband communications techniques, the apparatus changes the
parameters to provide narrowband communications at the minimum
acceptable power level.
Inventors: |
Krueger; Thomas E. (Kirkland,
WA), Astrof; Kenneth L. (Edmonds, WA) |
Assignee: |
Seattle Silicon Corporation
(Bellevue, WA)
|
Family
ID: |
22272383 |
Appl.
No.: |
08/099,058 |
Filed: |
July 29, 1993 |
Current U.S.
Class: |
375/149;
375/E1.002; 455/62; 455/68; 455/69 |
Current CPC
Class: |
H04B
1/707 (20130101); H04W 52/04 (20130101) |
Current International
Class: |
H04B
7/005 (20060101); H04B 1/707 (20060101); H04B
007/216 (); H04B 001/64 (); H04B 017/00 () |
Field of
Search: |
;375/1
;455/62,68,69,70 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Barron, Jr.; Gilberto
Attorney, Agent or Firm: Graybeal Jackson Haley &
Johnson
Claims
We claim:
1. A method for communicating between a first station and a second
station over a plurality of communication channels, the first
station including a first transmitter and a first receiver and the
second station including a second transmitter and a second
receiver, the second receiver at the second station sequentially
monitoring each of the channels in the plurality of communication
channels for communication signals transmitted by the first
transmitter at the first station for a predetermined period of
time, the method comprising the steps of:
a) initiating the establishment of a link of communication channels
between the first station and the second station, each of the
communication channels of the link being characterized by a
predetermined set of spread spectrum parameters, each parameter
taking a value in a range of values such that the communication
reliability of a communication channel changes monotonically as the
value of the corresponding parameter changes from one extreme of
the range to the other extreme of the range;
b) determining whether the link is established between the first
station and the second station;
c) if the link is not established, broadcasting communication
signals from the first transmitter to the second receiver over a
first one of the plurality of communication channels for a period
of time equal to the total period of time over which the second
receiver monitors the channels in the plurality of communication
channels;
d) if the link is not established after performing step c),
changing one of the parameters in at least one of the predetermined
sets of spread spectrum parameters at the first station and
returning to step b);
e) if the link is established after performing step c), changing
the one of the parameters in the at least one of the predetermined
sets of spread spectrum parameters at the second station;
f) monitoring the spread spectrum parameters associated with each
channel in the plurality of communication channels to determine
which channel in the plurality of communication channels is
characterized by spread spectrum parameters, at least one of which
has a critical value that establishes a communication reliability
that is the smallest possible greater than a predetermined
communication reliability between the first station an the second
station;
g) transmitting a communication signal from the first station to
the second station, the communication signal including information
identifying which channel in the plurality of communication
channels has a spread spectrum parameter that has a critical value;
and
h) causing the first and second transmitters and the first and
second receivers to change to the channel in the plurality of
communication channels that has the spread spectrum parameter that
has a critical value.
2. The method of claim 1 wherein the parameter in the plurality of
spread spectrum parameters is transmitter power.
3. The method of claim 2 wherein the parameter in the plurality of
spread spectrum parameters is sequence length.
4. The method of claim 1, further comprising the steps of:
i) adjusting a second parameter in the plurality of spread spectrum
parameters to reduce the communication reliability between the
first station and the second station to a critical value;
j) testing the link between the first station and the second
station;
k) if the link between the first station and the second station is
still established after performing step j), determining whether the
values of the first parameter and the second parameter in the
plurality of spread spectrum parameters both have critical values
that establish the communication reliability between the first
station and the second station;
1) if the values of the first parameter and the second parameters
in the plurality of spread spectrum parameters have critical
values, changing the communication mode between the first station
and the second station to communication in a single narrowband
channel;
m) if the link is not established after performing step l),
changing the second parameter to the value it had before performing
step i);
n) if the values of the first parameter and the second parameters
in the plurality of spread spectrum parameters are not both at
their respective critical values, returning to step i);
o) if the link between the first station and the second station is
not still established after performing step j), returning the value
of the second parameter in the plurality of spread spectrum
parameters to the value it had before performing step i); and
p) otherwise, establishing the values of the first and second
parameters in the plurality of spread spectrum parameters.
5. The method of claim 4 wherein the first parameter in the
plurality of spread spectrum parameters is transmitter power.
6. The method of claim 5 wherein the second parameter in the
plurality of spread spectrum parameters is sequence length.
7. The method of claim 4 wherein the second parameter in the
plurality of spread spectrum parameters is sequence length.
8. An apparatus for communicating between a first station and a
second station over a plurality of communication channels, the
first station including a first transmitter and a first receiver
and the second station including a second transmitter and a second
receiver, the second receiver at the second station sequentially
monitoring each of the channels in the plurality of communication
channels for communication signals transmitted by the first
transmitter at the first station for a predetermined period of
time, the apparatus comprising:
means for initiating a link of communication channels between the
first station and the second station, each of the communication
channels of the link using a predetermined set of spread spectrum
parameters, each parameter taking a value in a range of values such
that the communication reliability of a communication channel
changes monotonically as the value of the corresponding parameter
changes from one extreme of the range of the other extreme of the
range;
means for determining whether the link is established between the
first station and the second station;
means for broadcasting communication signals from the first
transmitter to the second receiver over a first one of the
plurality of communication channels for a period of time equal to
the total period of time over which the second receiver monitors
the channels in the plurality of communication channels, if the
link is not established;
means for changing one of the parameters in at least one of the
predetermined sets of spread spectrum parameters at the first
station if the link is not established;
means for changing the one of the parameters in the predetermined
set of spread spectrum parameters in the at least one of the
predetermined sets of spread spectrum parameters at the second
station;
means for monitoring the spread spectrum parameters associated with
each channel in the plurality of communication channels to
determine which channel in the plurality of communication channels
is characterized by spread spectrum parameters, at least one of
which has a critical value that establishes a communication
reliability that is the smallest possible greater than a
predetermined communication reliability between the first station
and the second station;
means for transmitting a communication signal from the first
station to the second station, the communication signal including
information identifying which channel in the plurality of
communication channels has a spread spectrum parameter that has a
critical value; and
means for causing the first and second transmitters and the first
and second receivers to change to the channel in the plurality of
communication channels that has the spread spectrum parameter that
has a critical value.
9. The apparatus of claim 8 wherein the parameter in the plurality
of spread spectrum parameters is transmitter power.
10. The apparatus of claim 9 wherein the parameter in the plurality
of spread spectrum parameters is sequence length.
11. The apparatus of claim 8, further comprising:
means for adjusting a second parameter in the plurality of spread
spectrum parameters to reduce the reliability of communication
between the first station and the second station;
means for testing the link between the first station and the second
station;
means for determining whether the first parameter and the second
parameter in the plurality of spread spectrum parameters both have
critical values;
means for changing the communication mode between the first station
and the second station to communication in a single narrowband
channel;
means for returning the value of the first parameter in the
plurality of spread spectrum parameters to the value it had
previously;
means for returning the value of the second parameter in the
plurality of spread spectrum parameters to the value it had
previously; and
means for establishing the values of the first and second
parameters in the plurality of spread spectrum parameters.
12. The apparatus of claim 11 wherein the first parameter in the
plurality of spread spectrum parameters is transmitter power.
13. The apparatus of claim 12 wherein the second parameter in the
plurality of spread spectrum parameters is sequence length.
14. The apparatus of claim 11 wherein the second parameter in the
plurality of spread spectrum parameters is sequence length.
15. An apparatus for communicating between a first station and a
second station over a plurality of communication channels, the
second receiver at the second station sequentially monitoring each
of the channels in the plurality of communication channels for
communication signals transmitted by the first transmitter at the
first station for a predetermined period of time, the first station
including a first transmitter and a first receiver and the second
station including a second transmitter and a second receiver, the
apparatus comprising:
a first circuit to initiate a link of communication channels
between the first station and the second station, each of the
communication channels of the link being characterized by a
predetermined set of spread spectrum parameters, each parameter
taking a value in a range of values such that the communication
reliability of a communication channel changes monotonically as the
value of the corresponding parameter changes from one extreme of
the range to the other extreme of the range;
a second circuit to determine whether the link is established
between the first station and the second station;
a transmitter circuit to broadcast communication signals from the
first transmitter to the second receiver over a first one of the
plurality of communication channels for a period of time equal to
the total period of time over which the second receiver monitors
the channels in the plurality of communication channels, if the
link is not established;
a third circuit to change one of the parameters in at lest one of
the predetermined sets of spread spectrum parameters at the first
station if the link is not established;
a fourth circuit to change the one of the parameters in the at
least one of the predetermined sets of spread spectrum parameters
at the second station;
a fifth circuit to monitor the spread spectrum parameters
associated with each channel in the plurality of communication
channels to determine which channel in the plurality of
communication channels is characterized by spread spectrum
parameters, at least one of which has a critical value that
establishes a communication reliability that is the smallest
possible greater than a predetermined communication
reliability;
a first transmitter circuit to transmit a communication signal from
the first station to the second station, the communication signal
including information identifying which channel in the plurality of
communication channels has a spread spectrum parameter that has a
critical value; and
a sixth circuit to cause the first and second transmitters and the
first and second receivers to change to the channel in the
plurality of communication channels that has the spread spectrum
parameter that has a critical value.
16. The apparatus of claim 15 wherein the parameter in the
plurality of spread spectrum parameters is transmitter power.
17. The apparatus of claim 16 wherein the parameter in the
plurality of spread spectrum parameters is sequence length.
18. The apparatus of claim 15, further comprising:
a seventh circuit to adjust a second parameter in the plurality of
spread spectrum parameters to reduce the reliability of
communication between the first station and the second station;
an eighth circuit to test the link between the first station and
the second station;
a ninth circuit to determine whether the values of the first
parameter and the second parameters in the plurality of spread
spectrum parameters are both at extremes which minimize the
reliability of communication between the first station and the
second station;
a tenth circuit to change the communication mode between the first
station and the second station to communication in a single
narrowband channel;
an eleventh circuit to change the second parameter to the value it
had previously;
a twelfth circuit to return the value of the second parameter in
the plurality of spread spectrum parameters to the value it had
previously; and
a thirteenth circuit to establish the values of the first and
second parameters in the plurality of spread spectrum
parameters.
19. The apparatus of claim 18 wherein the first parameter in the
plurality of spread spectrum parameters is transmitter power.
20. The apparatus of claim 19 wherein the second parameter in the
plurality of spread spectrum parameters is sequence length.
21. The apparatus of claim 18 wherein the second parameter in the
plurality of spread spectrum parameters is sequence length.
22. The apparatus of claim 18 wherein the first, second, third,
fourth, fifth and sixth circuits are a programmed arithmetic-logic
unit.
23. The apparatus of claim 18 wherein the seventh, eighth, ninth,
tenth, eleventh, twelfth and thirteenth circuits are a programmed
arithmetic-logic unit.
Description
FIELD OF THE INVENTION
This invention relates to a method and apparatus for promoting
communications, and more particularly, to a method and apparatus
for promoting spread spectrum communications.
BACKGROUND
Conventional communication systems typically operate on the premise
that concentrating communication energy to a narrow bandwidth
overcomes conflicts with other communication systems sharing the
same frequency band by avoiding the specific frequencies that the
other communication systems are using. Spread spectrum
communication systems have been developed to provide enhanced
communications capabilities by spreading the communication energy
over a relatively wide bandwidth, rather than concentrating the
communication energy in a relatively narrow bandwidth.
A spread spectrum communication system reduces conflicts with
conventional communication systems by having a very low energy
density at any particular frequency. Conflicts are reduced because
a conventional narrowband receiver tuned to a particular frequency
will preferentially respond to a narrowband signal at that
frequency (especially when using a frequency modulation technique,
due to the "capture efect"), while a spread spectrum communication
system receiver that is adapted to receive signals spread over a
relatively wide bandwidth (in a manner to be described below)
preferentially responds to spread spectrum communication
signals.
An important factor in the operation of a spread spectrum
communication system is the manner in which the communication
energy is distributed over the available (relatively wide)
frequency bandwidth. Spread spectrum communication systems use
conventional carrier modulation techniques to apply the information
to be transmitted to the carrier frequency. The information can be
in analog or digital form. If the information is supplied in
digital form, its rate is known as the data rate. After the first
modulation, however, a spreading code is then applied to the
information-modulated carrier [a "chipping rate") to spread the
communication energy over the wide bandwidth available to the
spread spectrum communication system.
There are essentially four ways in which the carrier is spread out.
These are frequency hopping, direct sequence, chirp, and time
hopping. In the frequency hopping technique, a pseudo-random list
of distinct channels is developed and the information-modulated
carrier is made to hop from one channel to the next according to
the list. In the direct sequence approach, a pseudo-random sequence
is mixed with the carrier in order to change the phase or frequency
of the information-modulated carrier at a very fast rate. If the
phase shifting (phase shift-keying) is accomplished by a balanced
mixer, and the pseudo-random sequence is a binary sequence, the
phase shifting is typically produced by shifting the
information-modulated carrier between 0 and 180 degrees (binary
phase-shift keying--BPSK). Quadrature phase-shift keying (QPSK), in
which the information-modulated carrier is shifted among four
different phases is another common technique of direct sequence
spread spectrum communications. Other forms of modulation are
quadrature amplitude modulation (QAM), frequency shift-keying
(FSK), and multiple phase shift-keying (MPSK). Chirp spread
spectrum causes the frequency of the information-modulated carrier
to be swept along predetermined frequency ranges. Time hopping
operates by causing the information-modulated carrier to be keyed
on and off at a very low duty cycle, in accordance with a
pseudo-random binary sequence. The spread of the signal is
established by the keying speed.
It is also possible to have hybrid spread spectrum communication
systems, in which desirable features of two or more of the four
most common spreading techniques briefly described above can be
combined to suit particular circumstances. Further details of
spread spectrum communication systems are given in "Spread Spectrum
Systems," by R. C. Dixon, John Wiley and Sons, New York, 1984;
"Spread Spectrum Techniques," by R. C. Dixon, IEEE Press,
Piscataway, New Jersey; and "Coherent Spread Spectrum Systems," by
Holmes, Wiley Interscience, New York, 1982. These references are
hereby incorporated by reference.
One important aspect of many common spread spectrum communication
systems is the development of the pseudo-random (PN) sequence which
is used to spread the information-modulated carrier signal.
Typically, the PN sequence chosen in a particular spread spectrum
communication system has desirable statistical properties which
allow the signals transmitted by the particular system to be
suitably distinguished from the signals transmitted by other
communication systems. In addition, choices of PN sequences can
affect speed of acquiring synchronization of the received signals,
which relate to the efficiency of initiating interpretation of the
received signals. Common PN sequences are m-sequences, Thue-Moore
sequences, and Gold sequences. Their choice and implementation are
discussed in "Shift Register Sequences," by S. Golomb, Aegean Park
Press, Laguna Hills, California, 1982. Excellent general references
to spread spectrum communication systems are "Spread-Spectrum
Applications in Amateur Radio," by W. E. Sabin, QST, ARRL,
Newington, Connecticut, July, 1983; "Spread Spectrum Theory and
Projects," 1993 ARRL Handbook, ARRL, Newington, Connecticut, 1993;
"The Spread Spectrum Concept," by R. A. Scholtz, IEEE Trans. on
Comm., IEEE, Vol. COM-25, No. 8, August 1977, Piscataway, New
Jersey; and "The Origins of Spread Spectrum Communications," IEEE
Trans. Comm., May 1982, pp. 822-854, Piscataway, New Jersey. Each
of these five further references is also hereby incorporated by
reference.
In commercial communications systems, spread spectrum techniques
inhibit the casual listener from deciphering transmitted digital
data. However, security is not normally a major goal in
implementing a digital data spread spectrum system, since the data
can easily be encrypted in software before it modulates the carrier
signal.
While spread spectrum systems distribute communication energy over
a wider bandwidth than narrowband communication systems and
consequently benefit an independent co-frequency receiver, they
also impair an independent adjacent frequency user. The amount of
impairment is dependent upon the relative strengths of the desired
and undesired signals at the receiver as well as various spread
spectrum parameters that describe each of the channels used in a
particular spread spectrum communication system. The relative
powers of the desired and undesired signals are affected by
transmitter and receiver antenna patterns and the relative
positions of the desired and undesired transmitters (also known as
the near/far problem).
In addition to reducing the effects of fading on frequency
modulation signals due to multipath interference, a spread spectrum
communication system reduces intersymbol interference due to
ghosting (a particularly important consideration at data rates
greater than about 1 megabit per second).
The Federal Communications Commission has allowed unlicensed
operation of wireless communication systems in the frequency ranges
of 902-928 MHz, 2400-2483.5 MHz, 5725-5850 MHz and 24.0-24.25 GHz
If such a system is a narrowband system, it is limited to a maximum
transmitted power of 0.75 milliwatt (mW) (although if the output
signal is adequately scrambled, above 1000 MHz, the maximum
transmitted power can be up to 100 mW). However, if the system is a
spread spectrum system, its output power is limited to a much
greater maximum of 1 watt (W).
What are unknown in the prior art are methods and apparatus for
modifying the transmitter and receiver modulation as well as the
transmitted power parameters to achieve the desired data rate of a
spread spectrum communication system while minimizing interference
to other operating wireless communication systems.
SUMMARY OF THE INVENTION
In one aspect, the invention is a method for communicating between
a first station and a second station over a plurality of
communication channels. The first station includes a first
transmitter and a first receiver and the second station includes a
second transmitter and a second receiver. The second receiver at
the second station sequentially monitors each of the channels in
the plurality of communication channels for communication signals
transmitted by the first transmitter at the first station for a
predetermined period of time.
The method comprises the steps a)-h). Step a) is to initiate the
establishment of a link of communication channels between the first
station and the second station. Each of the communication channels
of the link is characterized by a predetermined set of spread
spectrum parameters. Each parameter takes a value in a range of
values such that the communication reliability of a communication
channel changes monotonically as the value of the corresponding
parameter changes from one extreme of the range to the other
extreme of the range. Step b) is to determine whether the link is
established between the first station and the second station. Step
c) is to broadcast communication signals, if the link is not
established, from the first transmitter to the second receiver over
a first one of the plurality of communication channels for a period
of time equal to the total period of time over which the second
receiver monitors the channels in the plurality of communication
channels. Step d) is, if the link is not established after
performing step c), to change one of the parameters in at least one
of the predetermined sets of spread spectrum parameters at the
first station and returning to step b). Step e) is, if the link is
established after performing step c), to change the one of the
parameters in the at least one of the predetermined sets of spread
spectrum parameters at the second station. Step f) is to monitor
the spread spectrum parameters associated with each channel in the
plurality of communication channels to determine which channel in
the plurality of communication channels is characterized by spread
spectrum parameters, at least one of which has a critical value
that establishes a communication reliability that is the smallest
possible greater than a predetermined communication reliability
between the first station and the second station. Step g) is to
transmit a communication signal from the first station to the
second station, the communication signal including information
identifying which channel in the plurality of communication
channels has a spread spectrum parameter that has a critical value.
Finally, step h) is to cause the first and second transmitters and
the first and second receivers to change to the channel in the
plurality of communication channels that has the spread spectrum
parameter that has a critical value.
In another aspect, the invention is an apparatus for communicating
between a first station and a second station over a plurality of
communication channels. The first station includes a first
transmitter and a first receiver, and the second station including
a second transmitter and a second receiver. The second receiver at
the second station sequentially monitors each of the channels in
the plurality of communication channels for communication signals
transmitted by the first transmitter at the first station for a
predetermined period of time. The apparatus comprises means for
initiating a link of communication channels between the first
station and the second station, each of the communication channels
of the link using a predetermined set of spread spectrum
parameters. Each parameter takes a value in a range of values such
that the communication reliability of a communication channel
changes monotonically as the value of the corresponding parameter
changes from one extreme of the range of the other extreme of the
range. The apparatus also comprises means for determining whether
the link is established between the first station and the second
station. Further, the apparatus comprises means for broadcasting
communication signals from the first transmitter to the second
receiver over a first one of the plurality of communication
channels for a period of time equal to the total period of time
over which the second receiver monitors the channels in the
plurality of communication channels, if the link is not
established. Still further, the apparatus comprises means for
changing one of the parameters in at last one of the predetermined
sets of spread spectrum parameters at the first station if the link
is not established. Additionally, the apparatus comprises means for
changing the one of the parameters in the predetermined set of
spread spectrum parameters in the at least one of the predetermined
sets of spread spectrum parameters at the second station. Also, the
apparatus comprises means for monitoring the spread spectrum
parameters associated with each channel in the plurality of
communication channels to determine which channel in the plurality
of communication channels is characterized by spread spectrum
parameters, at least one of which has a critical value that
establishes a communication reliability that is the smallest
possible greater than a predetermined communication reliability
between the first station and the second station. Even further, the
apparatus comprises means for transmitting a communication signal
from the first station to the second station, the communication
signal including information identifying which channel in the
plurality of communication channels has a spread spectrum parameter
that has a critical value. Finally the apparatus comprises means
for causing the first and second transmitters and the first and
second receivers to change to the channel in the plurality of
communication channels that has the spread spectrum parameter that
has a critical value.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a direct sequence transmitter for use
in a spread spectrum communication system known in the prior
art.
FIG. 2 is a block diagram of a direct sequence receiver for use in
a spread spectrum communication system known in the prior art.
FIG. 3 is a block diagram of a direct sequence transceiver in
accordance with the present invention.
FIG. 4 is a flow chart of the overall method of the present
invention.
FIGS. 5A-5E are a detailed flow chart of the overall method of the
present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The overall spread spectrum system of the present invention can
achieve the data rate while minimizing interference to other
operating wireless systems through the use of various techniques.
These techniques include changing the transmitted power level PN
sequence, PN sequence length, and frequency channel. Preferred
embodiments of these techniques will be described in the following
detailed description in sufficient detail that will be
understandable to one skilled in the art of spread spectrum
communication system design. However, those skilled in the art of
spread spectrum communication system design will be able to specify
further embodiments of communication systems in accordance with the
concepts described in the following detailed description and
appended claims. Accordingly, the scope of the present invention is
to be limited only by the claims.
FIG. 1 is a block diagram of a direct sequence transmitter for use
in a spread spectrum communication system known in the prior art.
The direct sequence transmitter 10 includes a data (or digital
modulation) source 12, preamble and PN sequence select circuitry
14, an RF oscillator 30, a mixer 29, a wide band final amplifier 16
and a transmit antenna 18. The preamble and PN sequence select
circuitry 14 includes a PN sequence generator 20, a preamble
generator 22, and a clock 24 driving both the PN sequence generator
20 and the preamble generator 22. A switch 26 selects between the
output of the PN sequence generator 20 and the output of the data
source 12. The composite signal generated at the output by proper
choice of the signals by the switch 26 therefore constitutes a
modulated signal consisting of a selected preamble followed by the
data to be transmitted. The preamble includes information which can
be used to identify the source of the data, or the transmitter (or
both), as well as containing additional information concerning
conditions under which the data are to be transmitted.
The switch 26 transmits the selected output signal to a mixer 28.
The PN sequence is clocked at a much faster rate than the
modulation of the digital signal produced by the data source 12.
The mixer 28 mixes the selected output signal with the output of a
radio frequency (RF) generator 30 and, accordingly, produces a very
fast composite signal.
The output of the mixer 28 is then mixed with the output of the RF
oscillator 30 in the mixer 29 and the output of the mixer 29 is
amplified in the wide band final amplifier 16 before the resulting
signal is transmitted by the antenna 18.
FIG. 2 is a block diagram of a direct sequence receiver for use in
a spread spectrum communication system known in the prior art. The
direct sequence receiver 32 includes a receive antenna 34, an RF
amplifier 36, a mixer 38, a first RF oscillator 40, a despread
circuit 42 and a low-pass filter 44. The receive antenna 34
receives the transmitted spread spectrum signal, which is amplified
by the RF amplifier 36. The output of the RF amplifier 36 is then
demodulated in the mixer 38 by the output of the first RF
oscillator 40, to remove the carrier frequency from the received
signal and produce a signal at a baseband frequency. The
demodulated signal produced by the mixer 38 is processed by the
despread circuit 42 to produce the digital information contained in
the received spread spectrum signal.
The despread circuit 42 includes an output mixer 46, a baseband
mixer 48 and a baseband oscillator 50. In addition, the despread
circuit 42 includes a synchronization processor 52, a clock 54 and
a PN generator 56. The processing circuits in the despread circuit
42 constitute a correlator and mix the output of the baseband
oscillator 50 with the output of the PN generator 56. The
processing circuits in the despread circuit 42 then mix the output
of the baseband mixer 48 with the incoming baseband RF signal
produced by the mixer 38. The information in the received spread
spectrum signal is then contained in the digital signal produced by
the output mixer 46 and all interference received with the received
spread spectrum communication system signal is spread to noise.
The PN generator 56 is driven by the clock 54, which is kept
synchronized to the output of the mixer 38 by the synchronization
processor 52. The output of the output mixer 46 is filtered by the
low-pass filter 44 which removes some of the noise to which the
interference is transformed by the output mixer 46.
FIG. 3 is a block diagram of a direct sequence transceiver in
accordance with the present invention. The direct sequence
transceiver 58 includes a transmit antenna 60, associated
modulation and amplification circuitry 62, a signal processing
circuit 64, associated amplification and demodulation circuitry 66,
a receive antenna 68 and a signal source 70. The signal source
drives both the modulation and amplification circuitry 62 and the
amplification and demodulation circuitry 66. The signal processing
circuit 64 is controlled by a control microprocessor 72 and
receives and transmits control signals and data from and to a host
74 (not shown). The control microprocessor 72 is programmed
according to conventional programming practices well-known to those
skilled in the art of computer programming and according to design
principles well-known by those skilled in the art of spread
spectrum communication systems control. The signal processing
circuit 64 receives a frequency signal from a crystal 76 and
synchronization/tracking and demodulated digital data signals from
the amplification and demodulation circuitry 66.
The signal processing circuit 64 (which may comprise a programmed
microprocessor) performs various tasks, such as control tasks 78,
frequency control tasks 80, transmit data preparation tasks 82,
despreading tasks 84, and receive data preparation tasks 86. The
transmit data preparation tasks 82 include a scrambler 88 and a
transmit PN code generator 90. The despreading tasks 84 include a
receive PN code generator 92 and a tracking voltage controlled
oscillator (VCO) 94. The receive data preparation tasks 86 include
a descrambler 96.
To transmit a spread spectrum signal, the direct sequence
transceiver 58 receives the control and data signals from the host
74. These signals are respectively sent to the control tasks 78 and
the transmit data preparation tasks 82. The control tasks 78
receive a clock signal from the crystal 76 and produce control
input/output signals for use in control functions as well as use by
monitoring programs which do not affect the operation of the direct
sequence transceiver 58. The scrambler 88 and the transmit PN code
generator 90 in the transmit data preparation tasks 82 respectively
scramble the data to be transmitted (if desired) and generate the
transmit PN code. These two tasks then produce respective streams
of data that are combined in an exclusive-OR gate 98 and
transmitted to a binary phase-shift keyer (BPSK) 100 in the
modulation and amplification circuitry 62. Simultaneously, the
signal source 70 produces a carrier frequency signal that is also
transmitted to the BPSK 100. The BPSK 100 mixes the two signals it
receives and sends a properly modulated signal to a controllable
power amplifier 102 for amplification before transmission by the
transmit antenna 60. The controllable power amplifier 102 is
controlled by a control signal produced by the control tasks
78.
A spread spectrum signal intended for the transceiver 58 is first
received by the receive antenna 68. It is then filtered by the band
pass filter 104 in the amplification and demodulation circuitry 66
and then mixed with the output signal produced by the signal source
70 in the controllable mixer 106. The controllable mixer 106 is
under the control of the control I/O signals produced by the
control tasks 78. Following further filtering of the mixed signal
by the wide bandpass filter 108, the filtered signal is demodulated
by the despread code produced by the receive PN code generator 92,
in the demodulator 110. The output of the demodulator 110 is
further filtered in a selected bandpass filter 112 (selected in
accordance with signals produced by the control I/O task 78) and
the resulting signal processed by an envelope detector 114. The
envelope detector 114 produces a synchronization/tracking signal
and a BPSK-modulated digital data signal. The
synchronization/tracking signal is received by the despreading
logic 84 in the signal processing circuit 64, which uses the signal
to control the receive PN code generator 92 and the tracking VCO
94. The BPSK-modulated digital data signal is demodulated in the
demodulator 116 and the output transmitted to the receive data task
86 for further processing (including descrambling, if the received
signal is scrambled). The resulting digital data is then
transmitted from the signals processing circuit 64 to the host 74,
for further use.
FIG. 4 is a flow chart of the overall method of the present
invention. This method is implemented by a conventionally
programmed computer to achieve the aims of the invention. After
initiation (step 118), the method depends upon whether an original
link (i.e., a transmission connection between a first transmitter
(part of a transceiver such as that shown in FIG. 3) at a first
station and a second receiver (also part of a transceiver such as
that shown in FIG. 3)) is established (step 120). If an original
link cannot be established, the method establishes an alternate
original link (step 122) and selects an alternate PN sequence
generator, such as a Gold code generator (step 124). Regardless of
the circumstances of establishing an original link, the method then
establishes a minimum bandwidth which will allow the spread
spectrum communication system of the present invention to operate
acceptably (step 126). Then an appropriate narrowband channel is
selected (step 128).
FIGS. 5A-5E are a detailed flow chart of the overall method of the
present invention. FIG. 5A is a flow chart of the original link
establishment of step 120 in FIG. 4. The link is initiated (step
130) and the receiver and transmitter portions of the transceivers
at the first and second stations are all set at default sequence
length, Gold codes and transmit powers (step 132). Then the
transmitter at the first station broadcasts on a first channel for
a length of time equal to the number of channels in the spread
spectrum communication system, times a factor of four times the
total duration of time that the transceiver normal spends at each
channel in accordance with the chipping rate (i.e., the sequence
length times the period of each bit in the PN sequence). The
receiver at the first station listens for half of that time on the
first channel. Next the transmitter and receiver at the first
station repeat these steps for each successive channel until the
first receiver receives an acknowledgement signal from a
transmitter at the second station (step 134). During this time the
receiver at the second station continually listens for each channel
for a duration of time equal to the time that the receiver at the
first station listens.
If the receiver at the first station does not receive an
acknowledgement from the second transmitter (branch 136), the
transmitted power at the first transmitter is increased by one
programmable step (if possible to increase the transmitted power)
(step 138) and the program then returns to step 134. If the first
receiver receives an acknowledgement from the second transmitter,
the second transmitter thereafter transmits at the same output
power level as the first transmitter used to connect to the second
receiver.
FIG. 5B is a flow chart of the alternate link establishment process
of step 122 in FIG. 4. The link is initiated, as in step 130 of
FIG. 5A (step 140), and the transceivers at the first and second
stations are set to default spread spectrum parameters, as in step
132 of FIG. 5A (step 142). Next the first receiver monitors a
default channel for a length of time equal to twice the sequence
length times the duration of each bit (step 144). If the first
receiver receives a correlation flash (step 146), the first
receiver times out for a period of eight times the sequence length
times the duration of each bit (step 148) and then returns to step
144. If the first receiver does not receive a correlation flash
(step 150), the first transmitter transmits a request to respond
(step 152) and the first transceiver changes to receive mode from
transmit mode (step 154) .
If the first transceiver does not receive an acknowledgement signal
from the second transmitter (step 156), the transmitted power of
the first transmitter is increased one programmable step (if
possible) (step 158), and the method returns to step 152. If,
however, the first receiver does receive an acknowledgement signal
from the second transmitter, the second transmitter is programmed
to transmit at the same output power as the first transmitter (step
160).
Operation of the method then progresses to the Gold code selection
process shown in step 124 of FIG. 4. A flow chart of the Gold code
selection process is shown in FIG. 5C. In this step the first
receiver chooses a Gold code from a predetermined list and monitors
the received signals for a period of time equal to four times the
sequence length times the duration of each sequence bit (step 162).
If the first transceiver receives a correlation flash (step 164),
it then advances its choice of Gold code by one (step 166) and
returns to step 162. If the first receiver receives no correlation
flash (step 168), the first transmitter transmits an empty Gold
code to the second receiver (step 170), causing the receiver and
transmitter sections of both the first and second transceivers to
move to the empty Gold code (step 172). Thereafter, the first
transmitter transfers control to the receiver at the second station
and the first transceiver changes to receive mode from transmit
mode (step 174). If the first receiver does not receive an
acknowledgement signal from the second transmitter (step 176), the
first transmitter transmits to the second receiver to cause the
second receiver to return to the default Gold code (step 178) and
the program returns to step 162. If the first receiver does receive
an acknowledgement signal from the second transmitter (step 180),
the method then moves to the step of establishing the minimum
bandwidth (step 126 of FIG. 4).
FIG. 5D is a flow chart of the process of establishing minimum
bandwidth, shown in step 126 of FIG. 4. In step 126, the first
receiver reduces the sequence length to the next lower programmed
sequence length, or to a narrowband operation, if the power output
is less than 0.75 mW (step 182). If the receiver changes to
narrowband operation, the program moves to step 128 in FIG. 4 (step
184). Otherwise, the first receiver listens with the current
selection of Gold code for a time equal to four times the sequence
length times the duration of a bit (step 186). If the transceiver
receives a correlation flash (step 188), the first receiver
advances one Gold code (step 190). If all Gold codes result in a
correlation flash, the sequence selected return to a previous
longer sequence and Gold code (step 192). Otherwise, the method
returns to step 186.
If the receiver develops no correlation flash (step 194), the first
transmitter transmits a new sequence length and Gold code to the
second receiver (step 196) and the first and second transceivers
all move to the new sequence length and Gold code (step 198).
Thereafter, the first transmitter transmits to the second receiver
and then the first transceiver changes from transmit mode to
receive mode (step 200). If the first receiver does not receive an
acknowledgement signal from the second transmitter (step 202), the
first transmitter transmits to the second receiver to return to the
longer previous sequence length and Gold code (step 204) and moves
to the next operation, step 128 in FIG. 4. Otherwise, if the first
receiver an acknowledgement signal from the second transmitter
(step 206), the method returns to step 182.
FIG. 5E shows a flow chart for the narrowband channel selection
process, step 128 in FIG. 4. In this process, the first receiver
step through each channel, while remembering the channel with the
lowest RSSI (step 208). Then the first transmitter transmits to the
second receiver to go to the channel with the lowest RSSI (step
210) and the first and second transceivers both move to the
specified channel (step 212). Following this, the first receiver
transmits to the second receiver and the first transceiver changes
from transmit mode to receive mode (step 214). If the first
receiver does not receive an acknowledgement signal from the second
transmitter (step 216), the first transmitter transmits to the
second receiver to return to the longer previous sequence length
and Gold code (step 218), and the link has been completed (step
220). Otherwise, the first receiver receives an acknowledgement
signal from the second transmitter and the link has been completed
(step 220).
* * * * *